If everyday were like today...

I'm confused.

Weight loss calories are not an on/off switch, so I don't understand about losing at 1200 or less, but gaining at more than1200. For example, if one loses at a pound a week at 1200, they should lose half a pound a week at 1450, a quarter pound a week at 1575, and so forth . . . and maybe even a little faster than estimated at the higher intake number, because many people get a little more energetic and active in daily life when they add back calories, so can eat a little more to maintain than they might expect.

But that's not the answer to your question.

When MFP estimates your weight loss future, including in those "in 5 weeks" messages, it is using a statistical estimate, and assuming you have exactly the same average daily calorie burn of people who have your same profile settings (age, weight, activity level, and so forth). But almost no individual is exactly the same as the average of the whole group of similar-characteristic people. (Some are fidgety, some are passive/lethargic, they're genetically slightly different, and more.)

Most people will be close to the average of their statistical group, so MFP's estimates seem spot on to them. A few people are little farther off the average, and a very, very few can be quite far off the average.

From what you say, I'm thinking you've set your calorie goal manually, probably. But MFP is still using your profile settings (age, activity level, etc.) and the group averages when it gives you the "in 5 weeks" messages.

My "in 5 weeks messages" tell me I'll gain weight in situations where, even if I did exactly that same day's thing for 5 weeks, I know I'd lose weight, based on 4 years of calorie logging experience. So I ignore the message, because I know it's wrong, and I know why.

So, my advice is "ignore the message".

Best wishes!